Certain methods for making fabrics by bonding together fibers of two types of thermoplastic material are known. However, these conventional methods, and products made thereby, have particular disadvantages. Furthermore, the art of making non-woven fabrics is demanding; a method or product that is ideally suited for one application can be completely unsuitable for another application, even though, for example, the products share some (perhaps many, but not all) characteristics.
U.S. Pat. No. 4,904,523, discloses non-woven fabrics having multiple layers each comprising fibers that are dry laid. In a representative embodiment, a first layer comprises thermoplastic matrix fibers and a second layer comprises expensive binder fibers. The second layer is superposed on the first layer and the resulting composite is heated to a temperature that is lower than the melting or softening point of the matrix fibers but higher than the softening point of the binder fibers. Thus, the binder fibers comprising the second layer become adhered to the matrix fibers comprising the first layer. Unfortunately, certain areas of the fabric are subject to delamination because uniform bonding between the layers is difficult to achieve.
Other conventional methods for making such fabrics include forming non-woven webs of continuous thermoplastic fibers, such as disclosed in U.S. Pat. No. 4,769,266. The continuous fibers are bonded together by the application of heat to soften the fibers sufficiently to render them cohesive. In such thermal binding techniques, the web can be subjected to both heat and compression to increase interfiber contact. A disadvantage of such methods is that, when the fibers are bonded at substantially all points of interfiber contact, the resulting fabrics tend to be stiff. Also, achieving a uniform distribution in a fabric of continuous fibers, which is required for certain applications, has proven elusive. The latter is unfortunate because the characteristically uneven distribution of fibers in such fabrics reduces the mechanical strength of the fabric.
Finally, U.S. Pat. No. 3,756,908 discloses a non-woven flexible sheet consisting essentially of commingled fibrids of a non-fusible aromatic polyamide and short aromatic polyamide fibers. Fibrids are defined in this reference as small non-rigid film-like particles having two of their three spatial dimensions on the order of microns. The fibers and fibrids are disposed in a slurry and wet-laid on a paper-forming wire to form a web. The web is dried by passage over the surfaces of steam-heated cylindrical dryer cans at a temperature not exceeding 185.degree. C. Interfiber bonding is facilitated by subjecting the sheet to compaction by a heated calender. Unfortunately, this process requires expensive calendering equipment to achieve a satisfactory web strength for intended applications.
In view of the shortcomings of the prior art, there is a need for non-woven fabrics having a substantially uniform distribution of chopped fibers of synthetic polymeric materials, wherein the fibers are bonded to each other in the fabric without the use of extraneous binder materials.
There is also a need for such fabrics that exhibit, even at low basis weights (less than 1.0 oz/yd.sup.2), superior strength as measured by, for example, tear strength and burst strength.
There is also a need for such fabrics having excellent "hand" characteristics.
There is also a need for methods and equipment adapted for manufacturing the foregoing needed non-woven fabrics.